My car continues to behave funkily in terms of idle speed. Most of the time it's fine, but does the occasional fast idle when I pull up to a stop light or intersection. My indy thinks its the overvoltage protection relay, which he says he sees go bad on these cars a lot. Last time I had the car in the shop he cleaned up the contacts on the relay and put it back in. Seemed to reduce the frequency at which my car did its fast-idle dance, but now its starting to do it a lot again. I am not sure, by any means, that the overvoltage relay is the culprit ... I am pretty sure, though, that it is some component of the EDS.
Would there be any sense in replacing the overvoltage relay at this point? My line of thought is that if it's going to go at some point anyway, will it hurt to put a new one in? Ignoring the cost, would it be a good idea? Is it complicated to do, and are there any risks to it? I.e., is there the possibility that I might really booger something up if I try to do this? Would love some input of anyone who has done this.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

Both of the '87s I drive behave similarly in this respect . One is a bit more frequent and when cold it seems more apt to rev up when coming to a stop. I read a post about an updated EDS module, but I am not ready to spring for the $$$ for a new one UNLESS someone tells me that a new one definitely fixes these symptoms. Till then I live with it.
Sometimes I even shift into neut when coming to a long traffic light to keep the creep down and so I don't need to stomp on the brake pedal so hard!
As much as I like the EDS I really dislike the fact it misbehaves like this and I can't put a finger on the source of the problem because I don't have the factory test device, you know the one that sends pulses of varying length to a analog meter to indicate certain problems sources. Kinda hokey if you ask me. If you read the FSM you'll know what I mean.
As for the OVP relay, maybe I'll try that (I have a good spare) but it looks doubtful to me that that would be involved.
There is a position sensor on the rear of the IP that I always suspected might be involved if it gets a dirty resistance element, its like a big potentiometer I think.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

The EDS system is supplied power via the OVP relay. If the relay was failing it would drop power to the EDS system and you would lose the idle control speed function. This should result in an unassisted idle speed of 400-500 RPM, not a high idle speed.

The "analog thing" described by the prior poster is impulse counter. You can make on for about $10 in parts that are obtainable from any local Radio Shack. The EDS has a self diagnostic capability and the pulse counter is used to pull trouble codes from the system. Your indy should know what a pulse counter is and he should also know how to pull codes from the EDS system.

I'm still betting you have a binding throttle linkage somewhere.

Defective wiring to the engine coolant sensor will cause the symptom you describe.

Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]

Which sensor? It isn't by any chance the one over the t-stat housing, is it? Because that wire is chewed up quite a bit. According to the wiring diagram I have, that just goes to the dash readout but not the EDS ... originally I thought it might have been an EDS component, but when I asked my indy he said that particular wire just went to the gauge. I've seen a lot of different wiring diagrams, some more confusing than others, and some of them, to me, looked as though that wire might be involved in the problem. It's the wire that runs through the belt tensioner.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

I don't have the particular wiring diagram for you car, so you'll need to do a bit of checking, but you do have a coolant sensor that's connected, either directly, or indirectly through another component (i.e.: over-voltage relay, etc..) to your EDS system. That's the wiring that I am referring to.

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'95 E300 Diesel, 264,000 Miles. [Sold it]